The use of dead air as an insulating medium is well known, but known dead air insulating structures as applied to pipe insulation has various disadvantages such as being too bulky, difficult to engage and disengage from the pipe and requires structure in addition to the envelope containing the air for supporting the insulation around the pipe. Prior art pipe insulation employing dead air is also difficult to use with lengthy pipes without creating gaps in the insulation and is difficult to remove for inspection or repair of a given section of pipe.